Conventionally, metal substrates for industrial use are given a zinc phosphate treatment in the course of surface preparation, for improving corrosion resistance and adhesive property. Zinc phosphating agents, however, contain large quantities of phosphorus or nitrogen and also large quantities of heavy metals such as nickel, manganese and so on for improving the performance of resulting conversion coating films. In consequence, the chemical conversion treatment with such zinc phosphating agent adversely affects environment, and leaves great quantities of zinc phosphate or iron phosphate sludges after the treatment, giving rise to such problems as industry waste disposal.
Also for the purpose of improving corrosion resistance of industrial metal substrates, much space and time are required in a coating line for such processing steps as “degreasing—surface treatment—chemical conversion treatment—electrodeposition coating”.
JP 2003-155578A proposed a chemical treating agent free from such problems, for iron- and/or zinc-based substrates, which contains substantially no phosphate ion but contains zirconium ion and/or titanium ion and fluorine ion. However, the chemical treating agent for iron- and/or zinc-based substrates as described in JP 2003-155578A has a problem in that satisfactory corrosion resistance cannot be secured unless a coating film is applied by a coating step after the treatment therewith.
WO 02/103080A discloses a metal surface treating method which enables reduction in the time and space required for the treating steps by the use of a composition for metal surface treatment, which composition comprises (A) a compound containing at least one metal element selected from Ti, Zr, Hf and Si and (B) a fluorine-containing compound as a supply source of fluorine ion, whereby precipitating a surface treating film excelling in corrosion resistance on the metal surface containing at least either iron or zinc, and dispensing with a surface adjustment (leveling) step. This treating method using the surface treating composition of WO 02/103080A, however, is subject to a problem of failing to secure satisfactory corrosion resistance, unless a coating film is applied by a coating step after the treatment.
JP 2003-166073A and JP 2003-226982A disclose a surface treating composition for lubricated steel sheet, which contains (A) amine-modified acrylic resin, (B) at least one compound selected from phosphoric acid-derived compounds, hydrofluoric acid, metal hydrofluoric acid and metal hydrofluoric acid salt, and (C) at least one compound selected from molybdenum compound, tungsten compound and vanadium compound; and which, when coated on zinc-plated steel sheet to be used for automobile bodies or household electric appliances, can provide lubricated steel sheet excelling in press-shapability and corrosion resistance. However, the steel sheet which is treated with the surface treating composition as disclosed in JP 2003-166073A or JP 2003-226982A fails to show satisfactory corrosion resistance unless it is further given a chemical conversion treatment and applied with a coating film, and hence the invention cannot achieve reduction in steps or space-saving.
JP 2003-293161A discloses a polymer composition for metal surface treating agent, which comprises a specific copolymer having salicylideneamino group and amino group. The steel sheet treated with the polymer composition for metal surface treating agent as described in JP 2003-293161A again fails to show satisfactory corrosion resistance, unless a coating film is applied thereon by a coating step, and the invention cannot lead to reduction in steps or space-saving.
Furthermore, JP Hei 2 (1990)-282499A discloses a method for forming a coating film on apertures of coating object having complex construction such as an automobile body having apertures of not more than 500 μm in width, by cationic electrodeposition coating according to multistage energization method. The multistage energization method as described in JP Hei 2 (1990)-282499A is effective for covering spaces in a coating object having apertures of not more than 500 μm in width and improving the object's corrosion resistance but is still short of securing satisfactory corrosion resistance.
Furthermore, JP 2003-328192A discloses a method for forming a multilayer coating film by electrocoating using multistage energization method, a cationic electrodeposition paint containing plural emulsions among which the differences in quantity of electricity necessary for starting their respective precipitation are unified. This method, however, is yet incapable of providing sufficient corrosion resistance.
Disclosure of the Invention
The object of the present invention is to offer a method for forming a film excelling in corrosion resistance on a metallic substrate by multistage energization system, using a water-based film-forming agent which excels in electrocoatability and stability.
We have engaged in concentrative studies to now discover that the above object can be accomplished by applying a specific film-forming agent onto a metallic substrate by a multistage energizing method under specific conditions, and come to complete the present invention.
Thus, the present invention provides a method for forming a coating film on a metallic substrate by a multistage energization method at no less than two stages, characterized by using an electrodeposition bath which comprises a water-based film-forming agent (I) comprising 30-20,000 ppm in terms of the metallic mass of zirconium compound (A) and, as the base resin, an amino group-containing modified epoxy resin (B) which is obtained through reaction of an epoxy resin (a) with an amino group-containing compound (b), said epoxy resin (a) having been obtained through reaction of a diepoxide compound (a1) represented by the following formula (1):
                [in the formula, n is an integer of 1-50],a bisphenol epoxy resin (a2) and bisphenols (a3), wherein the first stage electrocoating is conducted by energizing at a coating voltage (V1) of 1-50 V for 10-360 seconds with the metallic substrate serving as the cathode, and the second and subsequent stage electrocoating is conducted by energizing at a coating voltage (V2) of 50-400 V for 60-600 seconds with the metallic substrate serving as the cathode, the difference between the coating voltage (V2) and the coating voltage (V1) being not less than 10 V.        
The water-based film-forming agent (I) which is used in the film-forming method of this invention excels in electrocoatability and stability and, according to the film-forming method of the present invention, coated articles of excellent corrosion resistance can be provided.
According to the film-forming method of this invention, furthermore, coating film of superior corrosion resistance can be formed even on a metallic substrate of which zinc phosphating step has been omitted. Omission of the zinc phosphating step solves the problem of the sludge disposal. The method also enables omission of the chemical conversion step, which leads to space-saving and time-shortening in a coating line.
The reason for the excellent corrosion resistance of the coating film formed by the film-forming method of this invention is thought to be that the film (F1) precipitated on the coated object side contributes to suppression of corrosion under the coating film and the film (F2) precipitated onto the surface side intercepts corrosion-promoting substances (e.g., O2, Cl−, Na+), each performing its allotted share of the two functions within the coating film structure.